Comparative adsorption capabilities of rubbish tissue paper-derived carbon-doped MgO and CaCO3 for EBT and U(VI), studied by batch, spectroscopy and DFT calculations.

Water pollution due to organic dyes and radionuclides is a challenging issue to the modern world. Cheap and efficient adsorbents are needed for their removal from wastewaters. Carbon-doped magnesium oxide (C-MgO) and calcium carbonate (C-CaCO3) were synthesized by the in situ hydrothermal treatment of Mg(OH)2 and Ca(OH)2 with carbon, and applied for the removal of eriochrome black T (EBT) at pH = 2.0 and uranium (U(VI)) at pH = 6.0. The Langmuir monolayer adsorption capacities of C-MgO (3.62 × 10-4 mol/g for EBT and 8.10 × 10-4 mol/g for U(VI)) were higher than those of C-CaCO3 (2.53 × 10-4 mol/g for EBT and 5.92 × 10-4 mol/g for U(VI)). The high adsorption capacity of C-MgO was also evidenced with DFT calculations which showed that the sorption energies (ΔE) of C-MgO for EBT (20.62 kcal/mol) and U(VI) (63.41 kcal/mol) were higher than those of C-CaCO3 for EBT (10.21 kcal/mol) and U(VI) (34.29 kcal/mol). In all cases, the electrostatic interactions were involved in the adsorption process. The sorption kinetic data followed pseudo-second-order kinetics. The results demonstrate that both C-MgO and C-CaCO3 are reusable and can be effectively applied for the elimination of EBT and U(VI) from wastewater.